Add tiles management

README.md

@@ -14,9 +14,15 @@ Trips and journeys are managed through the provided gRPC service and stored in t
 
 ### Tiles architecture and cache
 
-To reduce dependencies on the chosen database, we decided to manage geographical calculations directly in out code instead of relying completely on MongoDB geographical features.
+To reduce dependencies on the chosen database, we decided to manage geographical calculations directly in our code instead of relying completely on MongoDB geographical features.
 
-For opti
+We use a tiles system to retrieve relevant journeys easily. Each tile is defined by :
+
+- driver or passenger status of the journey
+- date of the journey
+- Tile ID in the tiles grid. The tiles grid is inspired by Valhalla https://github.com/Telenav/open-source-spec/blob/master/valhalla/doc/valhalla-tiles-basic.md. We use a level 0 grid
+
+Journeys are stored in each relevant tile (where a departure, destination or part of the polyline enters the tile)
 
 ### Search